The field of the invention is digital x-ray imaging systems, and particularly, x-ray fluorographic systems used in clinical cardiology to view cardiac vessel lesions.
In x-ray fluorographic systems the x-rays are received by an image intensifier which converts them to a bright optical image that appears on an output phosphor of the intensifier tube. This image is viewed with a video camera which converts the image to analog video signals, and these are digitized into frames of picture elements (pixels). These frames of digitized x-ray data are typically processed in a digital video processor to filter out undesired signals and enhance desired features. The resulting image data is then applied to a digital-to-analog converter which produces a video signal suitable for driving a CRT monitor.
The digitized frames of image data may be used in a number of ways by a cardiologist. As indicated above, the frames can be displayed immediately on the CRT monitor to provide the cardiologist with a cineangiogram or fluoroscopic view of the patient in real time. Such a mode of operation may be used, for example, when positioning a catheter in the patient's vascular system for treatment of a lesion. This mode requires that any processing performed by the digital video processor be carried out at a frame rate which does not cause the picture to flicker.
In the alternative, the acquired image frames may also be stored and played back by the cardiologist. For example, a series of image frames may be acquired for a complete cardiac cycle, digitized and stored. This series of image frames may be processed in a variety of ways to enhance specific features and then played back on the CRT monitor. The cardiologist may play the processed series of image frames back in real time to observe the vasculature during an entire cardiac cycle, he may step through each frame, or he may freeze the sequence on a particular image frame. These "off-line" modes of operation are employed primarily for location and diagnosis of lesions prior to any remedial procedure.
Current digital cardiovascular x-ray systems utilize digital image processing techniques to zoom and pan both static and dynamic cardiac images. In most cases, a statically zoomed image provides the most clinical information about cardiac vessel structure and composition. However, dynamic loops comprised of a series of x-ray frames yield valuable clinical information when played in real time or slow motion. When viewing such cineangiograms an interpolated image sequence provides increased spatial resolution, however, the cardiac motion forces the cardiologist to "track" the moving lesion or vessel. This movement not only distracts the cardiologist, but it also may force the doctor to slow the loop display rate or use a single-step mode. The problem is further complicated when the lesion or vessel of interest, moves in front of or behind other vessels during the cardiac cycle.
A solution to this problem is disclosed in U.S. Pat. No. 5,054,045 entitled "Coronary Tracking Display." In this system the cardiologist identifies a region in the first frame of the cineangiogram, and this region is tracked during each subsequent frame and made to remain stationary on the display screen. A vessel or lesion can thus be identified and easily followed during the play back of the cineangiogram loop. Other surrounding features are caused to move, but the identified region of interest remains stationary. In one embodiment the cardiologist examines each frame manually and identifies with a screen cursor the feature of interest to be followed. In the alternative, the feature of interest can be identified in the first frame and a means for automatically locating that same feature in subsequent frames may be employed. Although a number of techniques for automatically locating an identified feature in a set of x-ray images are suggested in the prior art, none provide a means which operates automatically and satisfactorily at real time in a commercially viable system.